I am fascinated by junk floating around stars. And no, not paparazzi, har har. I mean circumstellar material, literally gas and dust orbiting other stars. We see it around stars that are dying, we see it around stars being born, and we see it even after stars are well into their youth.

One such young’un is the bright and shiny HR4796, a star 240 light years away, with about twice the mass of the Sun. It’s known to be less than 10 million years old — compare that to the Sun’s age of 4.56 billion years; we’re 450 times older! — and has also been known for some time to have material around it in the shape of a ring. New observations by Japan’s huge 8.2 meter Subaru telescope have provided some of the sharpest views of this ring ever taken, and revealed some surprises.

Isn’t that lovely? [Click to enannulusenate.]

This picture is in the infrared, well outside what the human eye can see. The star itself is so bright it’s saturated, overexposed. That part of the picture is blocked out to make it easier to see details around it, but the star’s position is marked with a dot. The tendril-like structures radiating outward are not real, but are artifacts of the image processing techniques. You can ignore them.

The important thing is the ring itself, which is easy to spot. It’s almost certainly a circle, but we’re seeing it at an angle (about 13° from edge-on) so it looks like an ellipse. It’s huge; 22 billion km (14 billion miles) across, more than twice as wide as our entire solar system.

Again, the ring has been known for some time; for example it was seen in Hubble observations back in 2009 [NOTE: as astronomer (and my friend) Glenn Scheider points out in the comments below, HR 4706’s ring was seen long before 2009. I wasn’t clear when I wrote the previous statement; I was only alluding to one particular earlier observation, but it wound up sounding like it was the earliest such observation. My apologies for any confusion.]. But there is some new stuff here. For one, if you look along the long axis of the ring, you can see it looks fuzzy. That’s real! The ring is made of dust grains of various sizes, probably the result of bigger clumps colliding with each other and grinding themselves up into ever-smaller pieces (the authors of this reasearch (PDF) call this a "collisional cascade", my new favorite phrase for 2012). These grains of dust orbit the star, and the smaller ones get blown away from the star due to the pressure of its fierce light. Bigger grains are less affected, so they tend to stay in place.

So the main ring is made of bigger grains, while the smaller ones are blown back, forming a larger, extended ring. That fuzzier outer ring is fainter and harder to see, but we see it more easily along the long axis because of geometric effects (similar to why soap bubbles and giant shells of cosmic gas look like circles in space). So even though we only see a part of this outer ring, the fact that we only see it in those two spots is what makes it clear we’re seeing a ring at all! Funny how that works.

But there’s more. The ring isn’t centered on the star like you’d expect. Very careful analysis shows it’s offset by quite a bit, over 750 million kilometers! That’s 5 times the distance of the Earth to the Sun, about as far as Jupiter orbits the Sun in fact. I drew a line by eye along the axis of the ring, and you can see the star’s position is noticeably off.

How can that be? There are several possibilities, but these new observations rule many out, leaving one as being the most likely: the presence of a planet or planets closer in to the star, disturbing the orbits of the dust particles. As it happens, it’s hard to make a giant ring around a star like this without planets; they shepherd the grains into that tight annulus in the first place. So this observation that the ring is offset just adds more grist to the mill for the existence of these unseen planets.

And while no planets were seen in these images, that fact is useful too: you can put an upper limit to how bright the planets must be (if they were any brighter, they’d’ve been detected). At the age of the star, planets are still hot from their formation, and the more massive they are, the hotter they are, and the brighter they glow in the infrared. That means that by putting an upper limit on how bright they are, we can put an upper limit on how massive they are!

They found that farther out than about 22 billion km, there are no planets bigger than about 1.4 times Jupiter’s mass, and closer in, at 2.7 billion km, there are no planets bigger than about 17 times Jupiter’s mass. Lower mass planets can be detected farther out from the star, because they’re not overwhelmed by the star’s glare. Closer in, the planet has to be bigger and brighter to be seen. So while there must be planets circling this star, they have to be lower mass than those limits or else we’d see them.

Years ago, I worked on Hubble images of young stars with material like this around them (in fact I worked with some of the authors on this paper!) and it was one of my favorite projects to work on. The images were surreal and amazing, and it was always a thrill to get new data and see these gigantic systems for the first time, knowing no one had ever seen them as sharply and in such detail. But these new Subaru observation are as crisp as Hubble’s, because the technology improves all the time. We’ll be getting even better observations of these objects as time goes on, and that includes spying the planets currently invisible in all that muck. We’ve actually directly detected quite a few planets orbiting other stars, and that list will only get larger with time.

Tip o’ the occulting mask to reddit for the news. Image credit: Subaru Telescope, National Astronomical Observatory of Japan

Comments (29)

The Subaru press release (linked by the BA above) gives a dramatic explanation for why we may be seeing this disk still today :

Collisions between small solid bodies (“planetesimals”) left over from planet formation may continuously replenish the dust in these disks. The dust ring around HR 4796 A is such a debris disk

Which adds to the drama in the picture when you understand the dust is coming from colliding planets! (Or, okay, planetesimals ie. asteroid sized bodies, I don’t think we really know the size range of the colliding objects responsible do we?)

the authors of this reasearch (PDF) call this a “collisional cascade”, my new favorite phrase for 2012.

A new fave phrase of 2012 after just two days – not even that really? Crikey, what was your old one?

.. while no planets were seen in these images, that fact is useful too: you can put an upper limit to how bright the planets must be (if they were any brighter, they’d’ve been detected).

Excellent that they’ve done this here – have they also done that for the proplyds* around Vega, Beta Pictoris** and Epsilon Eridani** to name just three others? Because that’s the first I’ve heard of this and its a pretty neat way to narrow the masses of objects there.

Finally, my usual question because it enables me to understand the star and place it in perspective and imagination so well – what is HR4796’s spectral type – presumably an A or F class star?

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* Proplyds = protoplanetary disks – I gather that’s the technical term but for those unfamilr with it.

** Yes, I know we’ve already found exoplanets around both Beta Pictoris and Epsilon Eridani – but what about constraining the mass of those and extra ones based on that idea? Plus what about Vega’s case?

of how such a huge collision of worlds may appear in the case of our own pale blue dot. –

Finally, my usual question because it enables me to understand the star and place it in perspective and imagination so well – what is HR4796′s spectral type – presumably an A or F class star?

Aha! Should’ve checked that pdf paper linked by the BA first shouldn’t I? Here we are :

HR 4796 A is a young (8–10Myr; Stauffer et al. 1995),
nearby (72.8±1.7 pc; van Leeuwen 2007), A0V-type star
first identified as a debris disk system from an IR excess
observed with IRAS (Jura 1991). It has a co-moving M-type
stellar companion at a separation of 7.′′7 (Jura et al. 1993).

So it turns out HR4796 is a very Vegan star – sharing its spectral type of A0 V and additionally one with areddwraf companion.

Maybe that red dwarf fellow traveller for this Vegan-like star explains the off-set in the proplyd and is responsible for turning any forming exoplanets into eccentric orbiters – and ones that cause worlds there to collide?

Just on the off-chance that folks are wondering, chances of life – let alone intelligent life – evolving around HR4796 are minimal. Even without the planets here colliding or being on eccentric comet-like orbits. Sorry.

Why? Because this is a short-lived star with a spectral class of A0 V (found that info in the pdf in the OP) same as Vega and as hot, massive and short-lived as an A type Sirian star can before it scrapes into the blue-white high mass spectral class B.

This means HR4796’s main- sequence lifetime will be “only” around 300 million years and so any earth-like planets around it won’t – we think – have time to develop any indigenous lifeforms upon them before their Vega-like (& thus possibly very fast rotating and distorted) sun evolves into a red giant and fries the system.

Remember that life for most of our planet’s (pre~!)history was limited to microscopic bacteria and even the dinosausr were relative latecomers in the geological timescale.

Love these bright, short lived stars. Only 300 million years before it swells up to red giant status? Heck, that’s plenty of time for an invading species(us) to build space colonies around it and have some fun,,,(ie, no need for planets)

I know this star is in that “terrible ten-millions” phase of its life, but what can be extrapolated about our own system from this? The formation of the planets, yes, but what got left out there?

We really have no idea what’s orbiting Sol at 22 billion km, but Pluto is by no means the edge of the Hill sphere. I can’t wait until the full results of the WISE survery come out, but even that’s not going to find everything (tho it will find or rule out anything big).

@VinceRN:
I know you’re joking, but for anyone else who’s confused, HR names are from the “Bright Star Catalog”, orginally the “Harvard Revised” catalog (but for the last ~30 years maintained by Dr. Ellen Dorrit Hoffleit at Yale). The catalog contains data on every star you could possibly see with the naked eye; for that reason they’re popular astronomical targets.

If you account for the migration of the habitable zones as the star ages and brightens, it’s a lot less than 300 Myr. I can’t recall if scientists have found any clear evidence of life on the Earth at that age.

@7. Dragonchild:

I doubt we have all that much in common. A-type stars like HR4796 have formed (very quickly- less time for planets to form!) from much more massive clumps of material; their disks should be more massive and farther out (ice boils at greater distances from A stars than G stars)… Then there’s the slight difference that A-type stars aren’t thought to have convective outer layers, although what this does to their coronae and winds I couldn’t say.

This means HR4796′s main- sequence lifetime will be “only” around 300 million years and so any earth-like planets around it won’t – we think – have time to develop any indigenous lifeforms upon them before their Vega-like (& thus possibly very fast rotating and distorted) sun evolves into a red giant and fries the system.

Remember that life for most of our planet’s (pre~!)history was limited to microscopic bacteria and even the dinosausr were relative latecomers in the geological timescale.

More specifically, while prokaryotes can make multicellulars, even true ones with adhering and differentiated cells (cyanobacteria sheathed cell strands with nitrification cells), they don’t make complex trait multicellulars (tissues & body plans). That is the sole domain for eukaryotes, which have ~ 10^5 larger energy density from mitochondria symbionts for protein turnover, making and utilizing larger genomes.

The reason why mitochondrial symbionts can make the necessary simplified energy plants for eukaryotes is an oxidized atmosphere. The reason for life is to increase the hydrogenation of carbon to methane by catalyzing enzymes. It does so by taking oxygen from carbon dioxide and puts it onto iron. Even if we can tentatively trace back these Iron Banded Formations to ~ 4.28 Ga bp (billion years before present) in the arguable dating of the Nuvvuagituuq Greenstone Belt it took a long while before a a minute percentage* of that redox potential was freed as molecular oxygen.

As a matter of fact it took a decently wet and warm planet of Earth size some ~ 2 Ga (billion years), from ~ 4.5 – 4.3 Ga bp to ~ 2.3 Ga bp. So 0.3 Ga won’t make it, it is an order of magnitude too little time.

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* Of the redox potential ~ 1-2 % has gone to oxygen, ~ 75 % has oxidized iron, and the rest sulfur. The latter part is what makes up much of terrestrials, especially their core.

If you account for the migration of the habitable zones as the star ages and brightens, it’s a lot less than 300 Myr. I can’t recall if scientists have found any clear evidence of life on the Earth at that age.

Not clear evidence, but the question is interesting. [Astrobiology student here.]

For example, the hydrothermal vent abiogenesis theorists sets the average lifetime for modern vents as their upper limit for chemical evolution to free cells. That is some ~ 30 ka (thousand years).

The stated consensus is life “sometime before 3.8 Ga bp” (before present) which at times you see mentioned as “at 3.8 Ga bp” (see below on the LHB) or more correctly “sometime before 3.5 Ga bp” which is the most recent accepted date. (The ~ 3.8 Ga bp isotope ratios were argued, the fall back was several ~ 3.2 Ga bp observations of cell fossils, and at least one of the ~ 3.5 Ga bp observations of cells/stromatolites seems to be generally accepted. But that is a 2011 result, so the fallout is unclear to me.)

However contrary to what was firmly believed before 2010ish, actual models of the Last Heavy Bombardment (LHB) @ 4.1 – 3.8 Ga puts it recurrently as easily survivable. Cells proliferate and migrate faster than any realistic sterilizing bombardment can keep up with. And today we know of Golidilock survival zones ~ 1 km down the crust, that putatively can withstand even crust busters vaporizing the oceans. Life is a plague on a planet.

Similarly the Iron Banded Formations (IBFs) arguably but again recurrently dated to ~ 4.28 Ga bp of the Nuvvuagituuq Greenstone Belt may or may not be a result of photosynthetic bacteria. So the upper boundary for life gets pushed down over time.

Similarly, the lower boundary tend to get pushed up over time. Redating of the Apollo Moon rocks may put the Earth-Moon impactor as late as ~ 4.36 Ga bp.

Finally there are now coming out several gene family models that self-consistently puts important events from the Archaean Expansion (the gene family explosion) over the Great Oxygen Event (the oxygenation of Earth) to expansion onto land correctly in time. When I used them for a toy model gene family clock extrapolation in the fashion of physicists, I get that the first genome may date from ~ 4.31 Ga bp.

Assuming that the crust had to reform over some ten or tens of million of years if we accept the late Earth-Moon system age, that is rather consistent with hydrothermal vent abiogenesis theories, external observation can perhaps constrain abiogenesis to at most 40 Ma. So I would say that 300 Ma is more than enough for having biological evolution from chemical evolution, aka life.

[quote]That is the sole domain for eukaryotes, which have ~ 10^5 larger energy density from mitochondria symbionts for protein turnover, making and utilizing larger genomes.

The reason why mitochondrial symbionts can make the necessary simplified energy plants for eukaryotes is an oxidized atmosphere. [/quote]

I was going to suggest perhaps the ~100x increased flux from the star would give life forms the required energy for interesting life forms… then I remembered that’d only apply to a planet at 1 AU, and such energy would vaporize the oceans… You would need interesting chemistries to get around THAT problem.

So I take from your answer that we’re reasonably sure life took under 800 Myr to evolve on the Earth- 4.6 Gyr Earth age, followed by (perhaps) 3.8 Gyr old fossils. I guess the real questions I can see here are exactly when life formed on Earth, and did it/could it survive the formation of the Moon? If life really did originate in ~50 Myr (or even 100 Myr) that makes A-type stars much more interesting planetary hosts.

[quote]For example, the hydrothermal vent abiogenesis theorists sets the average lifetime for modern vents as their upper limit for chemical evolution to free cells. That is some ~ 30 ka (thousand years).[/quote]

Wow. Although I wonder how long vents on the early, more active Earth lasted… But I’m no geologist; for all I know the Earth settled down “quickly” on astronomical scales.

I am not sure I understand the evidence that the ring is not centered. It looks to me like the white line is does not exactly bisect the ellipse. It seems to be closer to the left side of the ellipse. If it were exactly centered I would think it would pass through the position of the star. What am I missing?

Actually, on closer inspection, I think that the star may be offset along the long axis of the ellipse, not the short axis. It appears to be about 1/30 or about 3% closer to the top end of the ring which is pretty close to the stated value.

That is incredibly cool.
Seriously, in contrast to its star, that dust ring is incredibly cool. It’s a wonder we can see it at all!
I love how astronomers and engineers are continually turning science fiction into science fact. I suppose that’s the case with any science, but I think that astronomy does it in a particularly spectacular way.
I only fear my expectations are going to get out of whack. At this rate, I expect to see an actual surface map of an exoplanet within 30 years. Who wants to call the odds?

A question about planet formation: if, when clumps collide with each other they usually grind themselves up in a collisional cascade, how do planets form? Do some materials tend to stick together more? (Rock as opposed to ice for instance) Or are there places near the star where dust grains are slightly warm – like candy left out on a warm day – so when they bump softly they stick?

@Jen, #20, a bit of both. Larger mass particles will tend to accumulate the smaller particles, eventually forming “rubble pile” asteroids/comets. Eventually, enough mass accumulates and gravitational heating, along with decay heat of radioisotopes become sufficient to liquify the rock, forming a rocky planet. Or sufficient gas accumulates around a small rocky body and forms a gas giant.
What we’re seeing in this ring is a nascent asteroid field, probably, as Phil mentioned, being shepherded by at least two planets.
Just as our asteroid belt would have been as the planets formed, eventually remaining as a sparse population of varying sized asteroids, unable to form a planet due to Jupiter’s disruptive influence.

So I take from your answer that we’re reasonably sure life took under 800 Myr to evolve on the Earth- 4.6 Gyr Earth age, followed by (perhaps) 3.8 Gyr old fossils

That is the official value yes, and that is reasonable. But really sure would be ~ 1 Ga (4.54 Ga bp Earth birth – 3.5 Ga bp relative consensus fossils).

And there are reasonable hopes that we can push that figure a lot more.

I guess the real questions I can see here are exactly when life formed on Earth,

I hope it can be constrained as I laid out in my #11 comment.

Gene clocks can be used to predict dates, but a) they have historically been spreading dates a lot (less so with modern techniques) b) they usually need an earlier dating and don’t do well on extrapolation. However, it seems a quick and dirty clock could be in the right neighborhood in this case. It hasn’t so long to go either, from 3.8 (or 3.5) Ga bp.

did it/could it survive the formation of the Moon?

No way! That impact removed the first crust, ocean and atmosphere, which had to be reformed by gravitational settling and cooling.

With the time frames coming out now, I wouldn’t be surprised if the Earth-Moon impactor sterilized a first biosphere. If so, the evidence of the Death From The Skies may still be found on the Moon…

Wow. Although I wonder how long vents on the early, more active Earth lasted…

Quite. The talk I cribbed from reformulated that as “or ~ 10^15 ms” IIRC, presumably illustrating the average reaction time for organic compounds under those conditions. And of course there were many vents, so many attempts, and some of the metabolic networks could spread in between.

But that is the order of it. Miller once calculated the abiogenesis times for his then soup theories before they too had to become proliferating cells, and that ended up ~ 12 Ma. This is, ironically, the estimated average time before a volume of sea water would circulate through a hydrothermal vent in the Archaean, breaking down any passive circulating “broth”.

You seem to have a keen physics intuition! Indeed, we are taught in astrobiology courses on planetary formation, as I remember it, that the size from small grains up to ~ 1 km large bodies is the unknown and unsolved “desert” where models rather fragment bodies than aggregate them. Smaller and larger bodies works well with traditional models from surface and gravitational forces.

However, I seem to remember having read somewhere recently that indeed a collisions with enough collisional sticking as you describe in your last suggestion solves the problem. I don’t think the hypothesis was that it comes from bodies thermal viscodynamic properties but how these aggregates works viscodynamically under the collisional process.

@Torbjorn, #23, quite correct, but neglecting eddy currents prevailing to create massive bodies that would be more proofed against typical impacts. Indeed, eddies CREATE planets by nature, the turbulence in the cloud impacted by whatever source of energy creating vortices that become significant masses.
I didn’t go into the math that gives migraines and “Goobered” it down a lot, but that is the primary point of nebula formation and planetary formation.
Indeed, I personally consider stellar ignition and output as the primary planetary formation point, escorting any preceding formation into existence. The double coupled math works out better (not migraine inducing math, but stroke inducing math there).

Phil’s appreciation for circumstellar debris systems (rings and others) is a passion I share (and central focus of my work). To elaborate just a bit on Phil’s comment on the HR4796A debris ring “the ring has been known for some time; for example it was seen in Hubble observations back in 2009″… Actually, with due respect to Phil, but to correct, HST did not observe HR4796(A) in 2009. We first observed it with Hubble using the NICMOS coronagraph in 1998 (see http://iopscience.iop.org/1538-4357/513/2/L127/pdf/985851.web.pdf ). At that time, it was only the 2nd debris disk ever observed by imaging starlight scattered by its circumstellar dust — Beta Pic being the 1st, of course. Since then we have been plodding along at a time-averaged rate of about one new light-scattering debris disk imaged every year (this is not an easy task!). Hubble has revisited HR 4796A three times since. First in 2001 with the STIS coronagraph giving us an exquisite high-resolution view of its debris ring (see: http://iopscience.iop.org/1538-3881/137/1/53/pdf/aj_137_1_53.pdf with observations discussed also in the context of the new Suburu data in the paper linked in Phil’s posting). Second, in 2005 we went back with NICMOS to get near-IR color information on the debris to combine with its optical scattering properties from the STIS data (see: http://adsabs.harvard.edu/abs/2008ApJ…673L.191D ) from which we speculatively inferred the possible presence of Tholins (radiationally evolved complex organic molecules) in the ring. Third, in 2008, we observed again with NICMOS but using its 2-micron coronagraphic polarimetry capability to learn more about the properties of the disk grains.

HR 4796A may not be the prototype circumstellar debris system (Beta Pic is), but may be an archytpe for many. It certainly continues to inform our still (in detail) uncertain understanding of the formation and evolution of exoplanetray systems.

Sorry Glenn (26), I should’ve been more clear. I only meant to link to that Hubble observation from a few years ago; I remember talking to you about the NICMOS ring observations when I was still at GSFC! I didn’t meant to imply that was the first observation of it.

Hi Phil, re #27… Your original post with the “E.g” was already clear that it wasn’t speaking to “firsts”, just priors — no worries or issues there. I was only trying to point out should anyone go searching in the MAST archive or publication for “Hubble observations back in 2009″ that there are none. The last (most recent) was from 2008 (but still in-work and unpublished, though the raw data are now in the public domain). A small point, I know, just was trying to clarify that. Not to quibble – as the gist of your posting was great! In detail, I suspect (maybe) the “2009 observations” you spoke of (and still do after your edit with parenthetical added) may actually refer to the 2001 observations that we published in 2009 (sometimes it just takes a while to cross all the i’s and dot all the t’s). Science isn’t always “instant”… BTW, if you (or any of your readers) will be at the AAS meeting next week, come by our poster paper with NEW STIS imaging of (other) circumstellar disks — should knock your sox off…

Finally, just noted your update (italics) refer with a typo to “HR 4709″ rather than HR 4796(A). No biggie – but you know how possessive we astronomers are about our “favorite” stars — and HR 4796A is indeed my personal favorite.

Technically the ring is around HR 4796A: the star is a member of a multiple system. HR 4796B is an M-dwarf star and it turns out that also the late-type star 2MASS J12354893-3950245 is probably a distant third member of the system (Kastner et al. 2008). Furthermore there are suggestions that 2M1235-39 may itself be binary, making the overall system a potential quadruple.